To meet the demand for future development, B3G (or 4G) mobile communication systems must support high-speed packet data transmission as high as tens of megabits per second or even hundreds of megabits per second. As wireless resources become intense increasingly, employing MIMO wireless transmission technique to exploit and utilize space resources as far as possible and raise the spectrum use rate and power efficiency has become a key in B3G mobile communication study.
Compared to traditional single antenna transmitting and single antenna receiving systems, the channel environment in MIMO wireless communication systems is more complex, and there are more factors that have influence on channel capacity; when the terminal is moving, the different types of channels between terminal and base station can be gone through and then its capacity can be changed more largely. In order to adapt to channel environment variations, different transmission methods can be used; if the channels are Rayleigh channels and relevant Rayleigh channels, space division multiplexing (SDM) and space-time coding transmission method can be used; if a strong direct path is available and a wire antenna array is used, the traditional beam shaping technique can be used; if the channels are static channels, spatial flooding transmission can be used. Apparently, to construct a practical system with a variety of techniques, the techniques must be efficient and reliable, and the implementation will be very complex. It is undoubtedly an important task to seek for a unified adaptive transmission method that is suitable for a variety of channel environments.
To implement adaptive transmission in a channel environment, the prior information of the channel must be utilized at the transmitting end. The prior information of channel can be complete or partial channel state information, wherein, partial channel state information is statistical information of the channel, including statistical information on interference and noise, Tx-correlated information, Rx-correlated information, mean value, and moving speed, etc. In a typical mobile communication environment, especially in a high-speed mobile communication environment, the channel varies rapidly with time, and the transmitting end is unable to ascertain complete channel state information; however, partial channel state information is approximately constant within a quite long period. Studies have shown: when the transmitter utilizes partial channel state information to transmit, the channel capacity and transmission reliability of MIMO system can be improved significantly.
Theoretical researches have shown: in a Tx-correlated Rayleigh fading channel environment, if the receiving end knows the channel state information and the transmitting end knows the Tx-correlated channel matrix, the optimal transmission scheme under the channel capacity maximization criterion is a characteristic mode spatial pre-filtering transmission scheme. At present, most of the efforts reported are for Tx-correlated Rayleigh flat fading channels; however, extended research on characteristic mode transmission in more generic channel environments has not been made extensively. Therefore, it is necessary to make further extended research, in order to implement a genuine unified transmission scheme applicable to a variety of channel environments.